Effect of O21, H2

11 O21, and N2

11 O21 ion-beam irradiation on the field

emission properties of carbon nanotubes

J. J. S. Acuna,1 M. Escobar,2,3 S. N. Goyanes,3 R. J. Candal,2,4 A. R. Zanatta,5

and F. Alvarez1,a)

1Instituto de Fısica “Gleb Wataghin,” UNICAMP, P.O. Box 6165 Campinas, SP, 13083-970, Brazil2INQUIMAE, FCEyN-UBA-CONICET, Ciudad Universitaria, Pabellon II, Buenos Aires, Argentina3Depto. Fısica, FECyN, UBA, Ciudad Universitaria, Pabellon II, Buenos Aires, Argentina4ECyT, 3iA, UNSAM, Campus Migueletes, San Martın, Pcia. Buenos Aires, Argentina5Instituto de Fısica de Sao Carlos-USP, P.O. Box 369, Sao Carlos 13560-250, Brazil

(Received 31 January 2011; accepted 23 April 2011; published online 15 June 2011)

The effect of O2þ, H2

þþO2þ, and N2

þþO2þ ion-beam irradiation of carbon nanotubes (CNTs) films

on the chemical and electronic properties of the material is reported. The CNTs were grown by the

chemical vapor deposition technique (CVD) on silicon TiN coated substrates previously decorated

with Ni particles. The Ni decoration and TiN coating were successively deposited by ion-beam

assisted deposition (IBAD) and afterwards the nanotubes were grown. The whole deposition

procedure was performed in situ as well as the study of the effect of ion-beam irradiation on the

CNTs by x-ray photoelectron spectroscopy (XPS). Raman scattering, field-effect emission gun

scanning electron microscopy (FEG-SEM), and field emission (FE) measurements were performed

ex situ. The experimental data show that: (a) the presence of either H2þ or N2

þ ions in the irradiation

beam determines the oxygen concentration remaining in the samples as well as the studied

structural characteristics; (b) due to the experimental conditions used in the study, no

morphological changes have been observed after irradiation of the CNTs; (c) the FE experiments

indicate that the electron emission from the CNTs follows the Fowler-Nordheim model, and it is

dependent on the oxygen concentration remaining in the samples; and (d) in association with FE

results, the XPS data suggest that the formation of terminal quinone groups decreases the CNTs

work function of the material. VC 2011 American Institute of Physics. [doi:10.1063/1.3593269]

I. INTRODUCTION

At present, it is recognized that carbon-based materials

in the form of nano-tubes, -onions, and -domes have an enor-

mous potential for field-emission devices applications.1,2

Carbon nanotubes (CNTs), in particular, are interesting

materials with various applications in materials science and

electronics.3,4 More recently, the incorporation of substitu-

tional atoms in the graphene network has been proposed as

an alternative route to modify its properties. The inclusion of

nitrogen, for example, is expected to change the electronic

properties of the material by acting as a donor impurity when

incorporated into a graphiticlike structure.1,5 However, the

effective control over the properties of either N-doped or

pure CNT’s is critical and requires removal defective struc-

tures typically present in these materials.6 Previous reports

confirm that Oþ ion-beam irradiation of nanotubes preferen-

tially eliminates carbon atoms bonded to highly reactive

parts of the material by forming volatile carbon oxide com-

pounds.7 Reports relating ion beam purification of nanotubes

is not common in the literature.1,7 Studies of graphite ion ox-

ygen beam erosion can help to give some clues about the

phenomenon involves in the oxygen nanotubes interaction.

However, these studies are generally performed by energetic

ions (>1 keV) in the attempt to understand the effect of O2þ

irradiation on graphite in fusion experiments, and conse-

quently, a straight forward comparison with our results is dif-

ficult.8 Although this limitation, our findings show that the

low energy O2þ bombardment produces volatiles compounds

(CO2 and CO) as reported with energetic O2þ ions probably

by eliminating defective C sites.

The etching process takes place because well-organized

structures such as fullerenes, nano-domes, -tubes, -onions,

and -horns containing graphitic planes are thermodynami-

cally more stable than amorphous (a-)C or other defective

structures.9–11 We note that, in spite of the extensive use of

etching purification process, the mechanisms leading to a

better material are not fully understood. Consequently, the

study of the microscopic mechanisms involved in the oxygen

ion-beam irradiation of nanostructured graphene-based

materials could improve our understanding as well as lead to

further control of their electronic properties.12 The effect of

oxygen etching can be seen, for example, in Fig. 1 where a

CNT structure presents flaws and dangling bonds. These

defects can react with oxygen species, which are either

eliminated or, when remaining attached, act as network

terminators.

Based on the above facts, the aim of this paper is to

investigate in situ the effect of O2þ, H2

þþO2þ, and N2

þþO2þ

ion-beam irradiation on the electronic properties of CNTs

deposited by CVD. The analysis of the physical-chemical

properties of the nanostructures produced by the ion-beama)Electronic mail: alvarez@ifi.unicamp.br.

0021-8979/2011/109(11)/114317/7/$30.00 VC 2011 American Institute of Physics109, 114317-1

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irradiation was studied by XPS in a ultrahigh vacuum (UHV)

chamber directly attached to the deposition and irradiation

chamber, i.e., the samples were analyzed by XPS without

atmospheric contamination.13 Afterwards, and in order to

study the changes in both structural and electronic properties

of the material, ex situ Raman scattering, electron micros-

copy, and field emission experiments were performed.

II. EXPERIMENTAL DETAILS

The CNTs are synthesized by CVD using acetylene as a

source of carbon on an in situ decorated catalyst Ni nanopar-

ticles (NPs) substrate. In fact, the substrate consisted of mir-

ror polished n-type (�15 X/cm2, <100>) silicon wafers

coated with TiN. The purpose of the TiN film (�100 nm

thick) is to avoid the formation of nickel silicide and it was

grown by ion-beam assisted deposition (IBAD).14 Briefly, a

high-purity titanium target (99.9995%) was sputtered by ar-

gon ions (1450 eV, 13 mA/cm2) in a nitrogen atmosphere

(partial pressure: 4.8� 10�2 Pa) at 650 �C. Afterwards, the

Ni NPs were deposited by sputtering a Ni target (99.9995%)

which was followed by annealing the sample for 5 min at

650 �C. Further details on the Ni island preparation can be

found elsewhere.15 After the catalyst NPs preparation, the

temperature of the substrate was raised to 700 �C and the

CNTs were grown during 5 min by introducing a gaseous

mixture of [H2]:[Ar]:[C2H2]¼ 1:10:6 sccm inside the prepa-

ration chamber (kept at 25 Pa). As we shall see below,

“carpet like” nanotubes are obtained from this procedure.

Three different series of nanotubes samples were then

irradiated in situ by O2þ, H2

þþO2þ, N2

þþO2þ ion-beams. The

chamber partial pressures due to the O2, H2, and N2 gases

feeding the Kaufman guns was maintained at 1.7� 10�3,

1.4� 10�2, and 1.9� 10�2 Pa, respectively. The nominal

energy of the ion-beam irradiation was fixed at 30 eV and

the current-density beam was �1.4 mA/cm2, i.e., the current

measured by the electronic ion gun instrument divided by

the area of the Kaufman exit grid of �3 cm diameter (Oxford

Instrument). Therefore, this grid determines the density of

current at the barley exit of the gun. Considering that in the

experiment the samples is located �20 cm from the gun exit

grid, the self ion repulsion spread up the beam to a diameter

of �10–12 cm lowering the density of current roughly one

order of magnitude, i.e., the beam at the sample is �0.14

mA/cm2. The selection of 30 eV ion energy is a compromise

between stability of the ion beam and low energy bombard-

ment of the sample to avoid strong sample damage. Finally,

each irradiation procedure is followed by annealing the sam-

ples at 400 �C for 10 min (in order to remove volatile com-

pounds) and afterwards transferred to the XPS analysis

chamber where their structural-chemical characteristics are

investigated.

The XPS spectra were taken using the Al Ka line

(��¼ 1486.6 eV) and a VG-CLAMP-2 electron hemispherical

analyzer14 providing a spectral resolution of �0.85 eV. The

atomic composition of the samples was determined by inte-

grating the core-level peaks, properly weighted by the photo-

emission cross section.16 The inelastic scattering background

contributing to the peaks associated with the electron core-

level spectra was subtracted by using Shirley’s method.17

Afterwards, the spectra were adjusted by a standard multiples

50%–50% Gaussian-Lorentzian peaks fitting procedure. The

ex situ, Raman spectra were obtained in the backscattering ge-

ometry by exciting the samples with the 488.0 nm at room-

temperature. Scanning electron microscopy (FEG-SEM GEM-

INI DSM 982) was employed for further characterization of

the nanostructures. The SEM images were essentially per-

formed with the main goal to observe possible changes due to

the irradiation treatment. Some TEM images (High-Resolu-

tion TEM, JEOL JEM-3010 URP) were obtained in few sam-

ples with the purpose to determine if the catalyze was on the

top or in the bottom of the nanotube, i.e., to discard that some

of the observed effects of the field emission experiments were

originated in metallic particles. Indeed, the TEM images show

that the nickel particles remain in the bottom of the nanotube,

i.e., they continue stacked in the substrate after sample

growth. It is important to remark that TEM experiments deter-

mining the number of nanotubes before and after irradiation

as well as their size and type (single wall, multi wall) could

bring interesting physical insight on the material properties.

At the present we have not systematic TEM images of the

samples and more work is necessary before drawing conclu-

sions regarding with the catalyst mechanism of the process.

At last comment regarding the catalyst. In accordance with

the TEM results, most of the nickel particles remain in the

root of the nanotubes. Therefore, the possible formation of

metallic compound with O, N, and H will not affect the field

emission results.

The field emission characteristics of the samples were

studied in a sphere-to-plane electrode configuration, with an

anode radii of 1.01 mm, at �22 �C, under vacuum conditions

(1.3� 10�4 Pa). The current versus voltage curves (I � V)

are obtained under different electrodes separation and nor-

malized. The I � V data are well represented by assuming a

Fowler-Nordheim emission mechanism. In the case of a par-

allel plane geometry, if d represents the distance anode-cath-

ode, the density of current J as a function of the electric field

E0 ¼ V=d is given by JðE0Þ ¼ aE20expð�g=E0Þ, where, V is

FIG. 1. (Color online) (a) Schematic CNT’s showing flaws and dangling bonds. (b) After ion-beam irradiation, oxygen species (open circles) are bonded to

these defects. (c) Further sample processing (such as thermal annealing, for example), eliminates very defective regions of the CNT and some oxygen acts as

bond terminators (open circles).

114317-2 Acuna et al. J. Appl. Phys. 109, 114317 (2011)

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the applied voltage, a ¼ ab2/�1, and g ¼ b/3=2b�1. Here, band / are the so-called enhancement geometrical factor and

work function of the material, respectively. The constants aand b are usually given, in the F-N equation, by a ¼ e3=8phand b ¼ 8p

ffiffiffiffiffiffiffiffi2me

p=3eh, where e and me are the charge and

electron mass, and h is the Plank’s constant. By direct substi-

tution one can obtain a¼ 1.54� 10�6 [A eV V�2] and

b¼ 6.83� 109 [eV�3/2 V m�1].

In the configuration of sphere-to-plane setup electrodes

such as those used in this paper, a correction of the F-N

equation is necessary and the following expression is

obtained for the I � V relationship:18,19

I ¼ 2prVag

V

d

� �2

exp�gd

V

� �: (1)

This equation is valid provided that r� d, where r is the

radius of the anode tip. The so-called linearized F-N form is

written as a function of d=V as:

lnId2

rV3

� �¼ ln 2p

ag

� �� g

d

V; (2)

where �g is the slope of the straight line represented by

Eq. (2). Defining I V=dð Þ ¼ Aeff J V=dð Þ, with r and d geomet-

rical known parameters, by fitting the experimental I � Vdata one can obtain the value of g and subsequently the

effective emission area given by:

Aeff ¼2prV

g¼ 2pbrV

b/3=2: (3)

As observed in Eq. (3) the effective emission area Aeff

depends on the applied voltage due to the fact that the anode

is curved.18

It is common to establish the relationship between / and

b from the slope of Eq. (2) or the interception of the curve to

the ordinate axis. However, previous reports suggest that the

former procedure does not always give reliable results.20,21

Therefore, in this paper we used the ordinate at the origin to

calculate the relationship / ¼ /ðbÞ, where / is given by:

/ ¼ 2pab3

bexp fð Þ

� �2=5

; (4)

where f ¼ lnð2pa=gÞ is the intercept defined by Eq. (2).

III. RESULTS

A. Morphological and structural characteristicsof the CNTs

Figure 2 presents FEG-SEM micrographs of all the stud-

ied samples. Figure 2(a) corresponds to the pristine sample.

From this picture (and the deposition time), the growth rate

was estimated to be �280 nm/min. The picture shows the

carpetlike CNTs in the back plane. The white parts observed

in Figure 2(a) correspond to CNTs that were peeled off and

moved from a different part of the sample. Figures 2(b), 2(c)

and 2(d) correspond to samples irradiated with O2þ,

H2þþO2

þ, and N2þþO2

þ ion-beam, respectively. These pic-

tures show that the main features of the original samples are

preserved in spite of the irradiation procedure.

Figure 3 shows the Raman scattering spectra correspond-

ing to samples irradiated with different ion-beam conditions.

The spectra show the so-called D-band (�1365 cm�1) and

G-band (�1590 cm�1) that are associated with the presence

of structural disorder (mode A1g) and graphite (mode E2g),

respectively.22,23 The Raman spectra present features that

depend on the ion-beam characteristics, which are related to

the oxygen incorporation.

FIG. 2. FEG-SEM images of CNT samples: (a) pristine, and (b) irradiated

with (c) O2þ, H2

þþO2þ, and (d) N2

þþO2þ.

FIG. 3. (Color online) Raman scattering spectra of CNTs irradiated in situwith O2

þ, H2þþO2

þ, and N2þþO2

þ ions. D and G stand for the disorder- and

graphitelike vibrational modes, respectively.

TABLE I. Atomic concentration (as obtained from XPS) of the studied

CNT samples: before and after irradiation.

Sample Carbon (at.%) Oxygen (at.%)

Pristine 100 0

O2þ irradiated 81 19

H2þþO2

þ irradiated 95 5

N2þþO2

þ irradiated 39 61

114317-3 Acuna et al. J. Appl. Phys. 109, 114317 (2011)

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B. Compositional and structural studies

The carbon and oxygen atomic concentration as

obtained from the XPS analysis is shown in Table I. First of

all, it must be said that, in the case of the N2þþO2

þ ion-beam

the nitrogen concentration remains below 0.5 at.%, i.e., the

limit of our detection technique. Also, we have noticed that

the presence of H2þ in the ion-beam diminishes the amount

of oxygen in the material. On the contrary, N2þ ions

enhanced the oxygen incorporation, which is even higher

than that provided by irradiating the samples with pure O2þ

ions. Considering that the XPS technique probes only the

most external surface of the samples (up to �5 nm) one can

assume that, most probably, these results correspond to the

tips of the nanotubes.

C. Field emission results

Figure 4 presents the ln(Id2/rV3) versus d/V curves for

all of the studied samples. As described in Sec. II this behav-

ior is characteristic of the Flower-Nordheim model, i.e., the

transport mechanism is a tunneling emission phenomenon.24

According to the literature, it is common to define the emis-

sion threshold field as the applied (macroscopic) electric

field (E0) necessary to produce a fixed current density.

However, this definition is not adequate considering that the

cathode geometry can influence the results.25 As a result, in

order to characterize the behavior of the electronic emission,

we propose that it is more reliable to calculate the current

density JL obtained when an arbitrary defined local electricfield, namely, EL¼ 1 V/nm, is applied. Here EL ¼ bE0, and

b, and E0 were previously defined. Experimental evidence

show that EL is typically a few V/nm, and it is significantly

higher than E0.25,26 Table III shows all the parameters

defined in Sec. II and obtained from fitting the Eq. (2) to the

experimental results displayed in Fig. 4.

IV. DISCUSSION

A. Morphological and material structuralcharacteristics

As observed from the Raman scattering spectra (Fig. 3),

the ion-bean irradiation treatments reduce the contribution

of the D-band while the G-band is not modified. In other

words, the disorder-associated vibrational modes are the ones

mostly affected by the treatment. The effect of the ion-beam

characteristic on the decreasing of the ID/IG ratio (Fig. 5) sug-

gests an efficient elimination of the more reactive parts of the

material—essentially dangling and wrong bonds. Moreover,

the Raman spectra present some broadening of the high-fre-

quency side of the G- and D-bands, most probably due to the

incorporation of oxygen in the CNTs (Table I).

FIG. 4. (Color online) Representation of ln(Id2/rV3) versus d/V curves, for

all studied CNT samples, before and after irradiation.

FIG. 5. ID/IG peak ratio obtained from the D- and G-bands (Fig. 3) after a

two Gaussian fitting procedure of the studied samples.

TABLE II. Summary of the different contributions present in the XPS spec-

tra deconvolution.

Band Binding energy (eV) Chemical bonding

PC1 284.4 6 0.1 aromatic CAC [1]

PC2 285.3 6 0.1 CAC sp3 type [1,2,13]

PC3 287.5 6 0.1 C @ O, CAO, and COO [1,2]

PC4 290.1 6 0.1 p-plasmons and/or shake-up [15]

PO1 530.3 6 0.2 >C @ O quinone [38]

PO2 531.7 6 0.2 C @ O in esters, amides and anhydrides [31]

PO3 533.2 6 0.2 AOH [1]

FIG. 6. (Color online) XPS spectrum and PCi fitting components associated

with the C1s core level. The spectrum corresponds to a CNT sample irradi-

ated with N2þþO2

þ ions.

114317-4 Acuna et al. J. Appl. Phys. 109, 114317 (2011)

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B. Effect of beam irradiation on the material chemicalstructure

As mentioned in Sec. III B, the fitting procedure of the

structure associated with the band due to C1s electrons yields

four bands: PCi with i¼ 1, 2, 3, 4 (Table II). All the studied

samples present C1s XPS spectra similar to the one shown in

Fig. 6, which corresponds to the CNT sample irradiated with

N2þþO2

þ ions. Here, PC1 (�284.4 eV) is attributed to CAC

bonds in graphitic sp2 configuration; PC2 (�285.3 eV) is

related to sp3 bonded carbon atoms; PC3 (�287.5 eV) is due

to carbon in carbonyl (C @ O<) and/or quinone groups; and

PC4 (�290.1 eV) is associated with the shake-up satellites

phenomenon, i.e., p! p* transitions in aromatic systems.16

Table IV contains the atomic percentage of each of these

contributions as well as their corresponding FWHMs.

Table IV roughly shows that independently of the ion-

beam composition, the sp2 bonds forming graphene are

eroded and, concomitantly, a relative increase of the sp3

bonds is observed. The increasing number of C @ O< and/or

quinone complexes are also observed in the irradiated sam-

ples. Finally, the presence of an almost constant contribution

of p! p* transitions (shake-up), typical of aromatic sys-

tems, suggests that the irradiation does not disrupt the conju-

gation of double bonds in the graphene skeleton.

Figure 7 shows the core levels due to O1s electrons

treated after irradiation with O2þ, H2

þþO2þ, and N2

þþO2þ

ions, as indicated in the figure. The fitting of the O1s core

level involves three main contributions: PO1 that is associ-

ated with quinone groups; PO2 that is attributed to carbonyl

(C @ O<), esters, amides and anhydrides; and PO3 that cor-

responds to OH groups (see Table II). For comparison pur-

poses, the relative concentration and FHWM of PO1, PO2,

and PO3 are shown in Table IV, as a function of the different

irradiation details. As can be seen, the data of Table IV sug-

gest that oxygen bond formation strongly depends on the

ion-beam characteristics. Moreover, two results are particu-

larly noteworthy: (a) the high contribution of C @ O quinoid

carbonyl groups (81.2%) in the samples irradiated with the

N2þþO2

þ beam (PO1); and (b) the high contribution of OH

groups attached to aromatic rings (PO3� 33.8 at.%) in the

CNT samples irradiated with H2þþO2

þ ions.

The material composition and formation of the different

structures discussed in connection with the XPS results

(Tables I, II, and IV) are a consequence of a complex inter-

play of chemical reactivity and kinetic of the process influ-

enced by the ion-beam bombardment. The energy of the ions

impinging on the surface of the samples is enough to break

C-C bonds, leaving dangling bonds to react with activated

species such as O2þ, N2

þ, and H2þ. The presence of H2

þ in the

ion-beam diminishes the amount of oxygen retained in the

material due its affinity with oxygen, explaining the low oxy-

gen concentration retained in the sample. At this point, it is

hard to explain why when irradiating with pure O2þ ions, the

amount of oxygen retained in the samples is smaller than in

the case of a N2þþO2

þ irradiation. This result suggests that

nitrogen species catalyze the incorporation of oxygen in the

material but more work is necessary to improve the under-

standing of this phenomenon.

The differences in the relative proportion of the func-

tional groups mentioned before can be explained based on

their chemical characteristics. The anhydride and esters

groups, responsible for the PO2 signal, are rich in oxygen

and can easily leave the carbon skeleton as volatile CO or

CO2. The simultaneous bombardment of the CNTs with N2þ

and O2þ produces esters or anhydride groups, which are im-

mediately eliminated as CO2 (or CO), leading to the produc-

tion of quinoid groups, responsible for the PO1 signal. On

the other hand, the presence of H2þ in the H2

þþO2þ ion-beam

decreases the concentration of oxygen species probably by

the production of H2O. Besides, the C @ O groups can be

FIG. 7. (Color online) XPS spectra associated to the emission of O1s elec-

trons in a CNT sample after irradiation with O2þ, H2

þþO2þ, and N2

þþO2þ

ions. The circles correspond to the experimental data, and the straight lines

to fitting functions (associated to C¼O ketone or quinine, C¼O carbonyl,

and -OH groups).

TABLE III. Summary of parameters obtained from Fig. 4.

Sample d [61 lm] g [61 V/lm] f Aeff [6102 lm2] / (b¼ 10) [60.5 eV] JL (b¼ 10) [60.1 mA/lm2]

Pristine 19 420 �32.29 6600 7.4 0.9

O2þ irradiated 22 593 �31.69 7100 5.8 1.3

H2þþO2

þ irradiated 15 856 �32.17 3640 7.0 0.1

N2þþO2

þ irradiated 17 457 �31.53 5440 5.4 1.5

114317-5 Acuna et al. J. Appl. Phys. 109, 114317 (2011)

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reduced to Ar-OH (aromatic alcohols) responsible for the

PO3 signal.

C. Electronic properties

The electronic emission parameters obtained by fitting

the experimental results shown in Fig. 4 to Eq. (1)–(4)

(Sec. II) are displayed in Table III. The b enhancement geo-

metrical factor was assumed 10—a factor generally accepted

for CNTs.27 As observed, the largest work function / was

achieved in the pristine samples. On the other hand, smaller

values were obtained in samples containing more oxygen af-

ter the irradiation treatment. Indeed, this finding is consistent

with the results reported by other researchers showing that

oxygen improves the electronic emission properties of

CNTs.12,28–32

The band associated with the O1s core level electrons is

located at a relatively low binding energy (530.3 eV), that

was recently attributed to the C¼O bond of quinoid

groups.33 In particular, within this group there is a functional

group, called benzoquinone, which contains two oxygen

atoms doubly bonded to carbon in terminal parts of the struc-

ture. As discussed before, the bombardment with the

N2þþO2

þ ion-beam contributes to the increment in the

concentration of quinoid groups at the tips of the CNTs. In

contrast, the H2þþO2

þ ion-beam reduces the quinoid concen-

tration. On the other hand, the Raman spectra indicate that

the irradiation slightly increases the width of the G-band

(Fig. 3), probably due to variations in the angles and length

of the sp2 type bonds, as well as broken bonds in aromatic

rings. We note that the treatment minimizing the structural

disorder (Fig. 5) is the one containing H2þþO2

þ ions in the

beam. Concomitantly, this treatment leads to less oxygen

incorporation in the material (Table I) and to the poorest

electron emission properties (Table III). Since for higher JL

electronic emission properties are better, one can conclude

that the material irradiated with N2þþO2

þ is improved, i.e.,

samples containing the highest oxygen concentration present

better emission properties. The relative high electronegativ-

ity of oxygen can help to understand these findings.7 The ox-

ygen atoms are bonded at the tips of the CNTs occupying

defects and mostly forming conjugated quinone groups.

Therefore, the formation of dipole terminators induced by

oxygen acting on delocalized p-electron may be responsible

by decreasing the effective work function of the material.

V. CONCLUSIONS

In this paper we presented a comprehensive study of the

effect of O2þ, H2

þþO2þ, and N2

þþO2þ ion-beam irradiation

on the field emission properties of CNTs. The CNTs were

grown by the CVD technique using Ni catalyst nanoparticles

deposited by ion-beam assisted deposition on silicon sub-

strates. The influence of the ion-beams composition irradia-

tion on the nanotubes structure was studied in situ by XPS.

Complementary information was gathered by ex situ Raman,

FEG-SEM. The characteristic parameters associated with

field emission properties of the nanotubes were estimated.

The work function of the material decreases when oxygen is

incorporated in terminator compounds, probably in defects

located in aromatic carbon rings present in the CNTs. The

treatment with N2þþO2

þ improves the field emission proper-

ties due to the formation of benzoquinonic functional group

(PO1 peak) by electronic delocalization (p-orbital) while the

treatment with H2þþO2

þ ion-beam increases graphitization.

In order to explain these differences we suggest that the

heavier N2þ ions induce defects that can bond to oxygen

atoms. The relative high electro negativity of the oxygen

increases the electronic density at the nano-tubes tips dimin-

ishing the work function of the material. The H2þ irradiation,

on the contrary, seems to improve the structural order and

prevents oxygen incorporation.

Finally, we propose an alternative criterion for evaluating

the electron emission of CNT’s which, presently, is based on

the electric field necessary to obtain an arbitrary electron

emission current. Indeed, considering that the cathode-anode

shape can influence the field emission results, we suggest that

it is more reliable to compare the relative emission goodness

of different samples by determining the current density

obtained when an arbitrary local electric field is applied.

ACKNOWLEDGMENTS

This work is part of the Ph.D. thesis of JJSA and was

partially sponsored by FAPESP (project 05/53926-1). JJSA,

ARZ, and FA are CNPq fellows. RJC, SNG and ME are

members of CONICET. Also, this work was supported by

UBA, Argentina (IP X191); CONICET (PIP 5215 & 5959),

TABLE IV. Atomic concentration (at.%) and FWHM (eV) of the C1s and O1s core levels, as obtained from the XPS analysis. The data correspond to CNT

samples irradiated with different ion-beam characteristics. The most probable origin of each PCi and POi contributions are indicated (brackets).

PC1

(sp2)

PC2

(sp3)

PC3 (C @ O<

and quinoid

carbonyl groups)PC4

(shake-up)

PO1

(quinone

groups)

PO2 (carbonyl,

esters, amides,

etc.)

PO3 (AOH

groups)

Sample

[C]

(at.%)

FWHM

(eV)

[C] (at.%) FWHM

(eV)

[C] (at.%) FWHM

(eV)

[C]

(at.%)

FWHM

(eV)

[O]

(at.%)

FWHM

(eV)

[O]

(at.%)

FWHM

(eV)

[O]

(at.%)

FWHM

(eV)

Pristine 60.5 1.2 21.2 2.0 1.6 2.0 15.5 4.9 — — — — — —

O2þ 55.0 1.2 24.1 2.3 5.1 3.1 15.7 5.7 42.9 1.3 51.4 2.3 5.7 1.2

H2þþO2

þ 56.2 1.2 26.0 1.8 3.3 1.6 14.3 5.0 13.4 1.7 52.7 2.3 33.8 3.6

N2þþO2

þ 53.4 1.2 23.9 3.6 10.4 5.7 13.1 10.0 81.2 1.7 15.6 1.5 3.2 1.3

114317-6 Acuna et al. J. Appl. Phys. 109, 114317 (2011)

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ANPCyT (PICT 10-25834 & 06-10621), Argentine-Brazilian

project CAPES – SPU 011/02. The authors are grateful to

Mr. C. Piacenti for technical assistance.

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